Hollow fiber membranes are widely used in the field of membrane separation, such as reverse osmosis, ultra-filtration, microfiltration, membrane contactors and membrane reactors separation. Common hollow fiber membrane materials include cellulose acetate, polyvinyl chloride, polypropylene, polyethylene, polyacrylonitrile, polysulfone and poly(ether sulfone). There are different requirements for hollow fiber membranes in different applications, such as membrane pore size, porosity, mechanical property and corrosion resistance. Polytetrafluoroethylene has powerful other good characteristics, acid and alkali resistance, oxidation resistance, microorganism resistance, high and low temperature resistance and other good characteristics. The polytetrafluoroethylene hollow fiber membrane has significant application value in the fields of special filtration, membrane contactors, membrane reactors, etc. for it also has advantages of high strength and flux in addition to the goods characteristics of polytetrafluoroethylene.
One method for processing the polytetrafluoroethylene hollow fiber membrane is that: polytetrafluoroethylene resin is used for paste extrusion under the action of lubricants to obtain a polytetrafluoroethylene tube blank; after removal of lubricants (removal of lubricant), the polytetrafluoroethylene tube blanks is stretched and sintered into a polytetrafluoroethylene hollow fiber membrane with micropores on the hollow fiber wall. Stretching is a key link in forming micropores, controlling micropore size and controlling the hollow fiber membrane structure, but there are few reports and patents concerning this aspect.
With regard to the link of extrusion, polytetrafluoroethylene tube blanks are generally extruded by using a push-compression machine whose extrusion mould comprises a cone mould, a die and core (as described in Fluoroplastic Processing and Application, Chemical Industrial Press, first edition and first printing in Beijing in July 2010, Page 152-154, Qian Zhimian, Bao Yongzhong, et al.) and the structure is as shown in FIG. 1. In practical use, the core is generally 1-3 mm longer than the die at the lower end.
One tube blank is extruded from one extrusion mould of the device mentioned above. ZL201010185858.1 discloses a mould for extruding and molding expanded polytetrafluoroethylene tubes which can extrude several tube blanks.
Patents related to stretching in the world include U.S. Pat. No. 4,250,138, which discloses a polytetrafluoroethylene hollow fiber stretching device. The device is similar to stretching devices in the form of seamless metal tubes and stretches polytetrafluoroethylene by means of a round-hole mould and mandrel.
U.S. Pat. No. 4,496,507 discloses a heating grooved roller stretching device in which heating rollers are adopted for controlling the temperature. The device stretches one hollow fiber for one time and tends to crack and collapse hollow fibers in stretching.
Patent ZL201010508798.2 discloses a polytetrafluoroethylene hollow fiber-stretching device, which is a device designed for removing lubricant and stretching fibers and comprising guide rollers, stretching rollers and cooling rollers. Patent ZL201010504784.3 discloses a method for controlling the pore size of polytetrafluoroethylene hollow fiber membranes, which controls the pore size by applying water dispersible fluorine-containing dispersion concentrates on the surface of polytetrafluoroethylene hollow fiber membranes.
In sum, under conventional techniques, stretching is performed along the length of tube blanks no matter what kind of stretching devices or means are used.